Method of cleaning ultrapure water supply system

ABSTRACT

A cleaning method is provided for an ultrapure water supply system having an ultrapure water production apparatus connected to a point of use of ultrapure water via a passage. In the cleaning method, a basic solution, for example, is circulated through the system to change the surface potential of fine particles in the system from an opposite to the same polarity as that of elements constituting the system, thereby facilitating the removal of the fine particles, and the fine particles are then discharged from the system to outside together with the basic solution. The cleaning method has excellent cleaning capability and also makes it possible to shorten the rinsing time required for removing the residual constituent of the cleaning solution.

TECHNICAL FIELD

[0001] The present invention relates to a method of cleaning anultrapure water supply system, and more particularly, to a method ofcleaning a system for supplying ultrapure water used in semiconductormanufacturing process etc.

BACKGROUND ART

[0002] As for ultrapure water used in the cleaning step of semiconductormanufacturing process or the like, an ultrapure water supply system isknown which circulates ultrapure water between an ultrapure waterproduction apparatus and a point of use. In such a system, the ultrapurewater produced by the ultrapure water production apparatus is suppliedthrough a pipe to the point of use, and the remaining part of theultrapure water that was not used in the point of use is returnedthrough a different pipe to the ultrapure water production apparatus.

[0003] The ultrapure water used in the semiconductor manufacturingprocess or the like is required that it should not contain fineparticles, organic matter and inorganic matter. Specifically, theultrapure water is expected to meet requirements of, for example, theresistivity being 18.2 MΩ·cm or more, fine particles contained being1/mL or less, viable cells contained being 1/L or less, total organiccarbon (TOC) contained being 1 μg/L or less, silica contained being 1μg/L or less, metals contained being 1 ng/L or less, and ions containedbeing 10 ng/L or less.

[0004] Thus, also in the above ultrapure water supply system, theultrapure water supplied to the point of use must satisfy these waterquality requirements. However, external fine particles or particlesproduced inside the system can become mixed with the ultrapure water,causing deterioration of the water quality.

[0005] For example, as the ultrapure water keeps circulating through theultrapure water supply system, the surfaces of a filtration membrane,pipes, etc. of the system are worn away, and the worn-off substances(fine particles) become mixed with the ultrapure water. Also, where thesystem has been stopped for a long term, fine particles such as deadbacteria or iron dust become mixed with the raw water or ultrapure waterstagnating in the system. Further, in the case of a newly installedultrapure water supply system, various kinds of fine particles remainstuck on the surfaces of the filtration membrane and pipes which arecomponent elements constituting the system. Also, while the system isunder construction, fine particles such as dust in the air, silica andaluminum enter the system and adhere to various parts of the system.When the newly installed system is set in operation, therefore, suchfine particles adhering to the interior of the system become mixed withthe ultrapure water circulating through the system.

[0006] Thus, in order to remove such fine particles, the ultrapure watersupply system needs to be cleaned prior to the start of operation aswell as at regular intervals of time. Conventionally, warm water orwater containing hydrogen peroxide is used to clean the system. However,in the case of warm water, for example, the cleaning capability is lowand it is probable that fine particles in the system cannot besatisfactorily removed. In Unexamined Japanese Patent Publication(KOKAI) No. 7-195073 is proposed a cleaning technique using alcohol withhigh cleaning capability. To remove fine particles to a satisfactorylevel, however, it is essential to use alcohol having relatively highconcentration (about 10 to 80%). Since alcohol, if left in the system,deteriorates the quality (increases the TOC) of the ultrapure waterproduced by the system, a residual alcohol removing operation (rinsing)must be performed, which, however, prolongs the overall cleaning time.

[0007] Such a long cleaning time lowers the operating efficiency of theultrapure water supply system as well as of the plant using the system,and therefore, the cleaning time should desirably be shortened.Especially in the case of a newly installed ultrapure water supplysystem, a set-up or preparatory time required for the system to produceultrapure water meeting the requirements is usually as long as a wholemonth, and accordingly, there has been a demand for techniques capableof shortening the set-up time.

[0008] A cleaning method is also known in which, when a filtrationmembrane constituting the ultrapure water production apparatus ismanufactured or attached to the apparatus, the filtration membrane iscleaned using ultrapure water to an extent such that the resistivity andTOC of the ultrapure water fall within respective allowable ranges. Withthis method using ultrapure water, fine particles that affect theresistivity or TOC can be removed, but it is probable that other kindsof fine particles are not satisfactorily removed.

DISCLOSURE OF THE INVENTION

[0009] An object of the present invention is to provide a cleaningmethod capable of satisfactorily cleaning an ultrapure water supplysystem and component elements thereof in a short period of time.

[0010] To achieve the object, the present invention provides a cleaningmethod for cleaning at least part of an ultrapure water supply systemhaving an ultrapure water production apparatus connected to a point ofuse of ultrapure water via a passage. The cleaning method of the presentinvention comprises the steps of: (a) changing surface potential of fineparticles present in the at least part of the ultrapure water supplysystem; and (b) discharging the fine particles from the at least part ofthe ultrapure water supply system to outside.

[0011] Fine particles in the ultrapure water supply system canoccasionally be charged with electricity. If the surface potential ofsuch charged fine particles is opposite in polarity to that of elementsconstituting the system, the fine particles adhere to the systemelements due to electrostatic attractive force acting between the systemelements and the fine particles, making it difficult to remove the fineparticles. In the method according to the present invention, the surfacepotential of the fine particles is changed, preferably into the samepolarity as that of the system elements, to eliminate the electrostaticattractive force acting between the fine particles and the systemelements, preferably to produce electrical repulsive force between thefine particles and the system elements, so that the fine particles canbe removed with ease. While in this state, ultrapure water, for example,is caused to flow through the system, thereby to discharge the fineparticles from the system to outside. According to the presentinvention, therefore, the whole or appropriate part of the ultrapurewater supply system can be cleaned satisfactorily in a short time. Also,it is possible to shorten the set-up time of a newly installed system.

[0012] Preferably, in the step (a), the fine particles are made tocontact with a basic or alkaline solution or a solution of surfactant.

[0013] According to this preferred embodiment, by making the fineparticles contact with the basic solution or the solution of surfactant,the surface potential of the fine particles can be changed without fail.Also, even if the solution (cleaning liquid) used has a lowconcentration of base or surfactant, the solution can produce asatisfactory effect of changing the surface potential of the fineparticles. Where a low-concentration solution is used for the cleaning,the constituent of the solution remaining in the cleaned system, thatis, the base or the surfactant, is small in quantity. As a consequence,there is a small possibility that the TOC of ultrapure water produced inthe system after the cleaning will increase due to the residualconstituent of the solution. Also, in cases where additional cleaning(rinsing) is performed using ultrapure water, for example, to remove theresidual constituent of the solution, the cleaning time may be short inlength, thus permitting the system cleaning as a whole, which includesthe cleaning by means of the cleaning liquid (solution) and the cleaning(rinsing) by means of the ultrapure water, to be completed in a shortperiod of time. In the step (b) of the preferred embodiment, the basicsolution or the solution of surfactant, which was made to contact withthe fine particles, is merely discharged from the ultrapure water supplysystem, for example. Alternatively, after the solution is discharged,ultrapure water for rinsing is introduced into the system and then thefine particles are discharged from the system to outside together withthe rinsing ultrapure water.

[0014] According to the present invention, preferably in the step (a),not only the surface potential of the fine particles is changed but alsophysical force is applied to the fine particles.

[0015] In this preferred embodiment, with the surface potential of thefine particles changed so as to eliminate the electrostatic attractiveforce acting between the fine particles and the component elements ofthe ultrapure water supply system or to produce electrical repulsiveforce between the two, physical force is applied to the fine particles,whereby the fine particles can be removed more easily.

[0016] Preferably, in the step (a), the basic solution or the solutionof surfactant is caused to flow through the at least part of theultrapure water supply system at a flow velocity of 0.5 m/sec to 2.0m/sec.

[0017] According to this preferred embodiment, when the basic solutionor the solution of surfactant flows through the system, the solutioncontacts with the fine particles in the system and, in addition, appliesphysical force thereto, thus promoting the removal of the fineparticles.

[0018] Alternatively, in the step (a), with the basic solution or thesolution of surfactant kept in contact with the at least part of theultrapure water supply system, the solution is applied withsmall-amplitude vibration. According to this preferred embodiment, thesolution contacts with the fine particles in the system and changes thesurface potential thereof, and in addition, small-amplitude vibration ofthe solution is transmitted to the fine particles to apply physicalforce thereto, whereby the removal of the fine particles can bepromoted.

[0019] Preferably, the basic solution is an aqueous solution of ammoniaor ammonium salt, or an aqueous solution of alkali metal hydroxide, or amixture of an aqueous solution of ammonia or ammonium salt and anaqueous solution of alkali metal hydroxide. Alternatively, the basicsolution may be pure water or ultrapure water in which alkaline gas isdissolved.

[0020] The basic solution used in this preferred embodiment can changethe surface potential of the fine particles without fail, thusfacilitating the removal of the fine particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic diagram showing an ultrapure water supplysystem according to one embodiment of the present invention;

[0022]FIG. 2 is a chart showing surface potentials of fineparticle-constituting substances and system-constituting materials as afunction of pH;

[0023]FIG. 3 is a chart showing the number of fine particles inultrapure water used in rinsing after system cleaning, as a function ofthe flow velocity of a cleaning liquid used in the system cleaning;

[0024]FIG. 4 is a schematic diagram showing an ultrapure water supplysystem according to a working example of the present invention;

[0025]FIG. 5 is a graph showing time-based changes in the number of fineparticles in the ultrapure water according to examples and a comparativeexample after the cleaning of the ultrapure water supply system;

[0026]FIG. 6 is a graph showing time-based changes in the TOC in theultrapure water according to the examples and the comparative exampleafter the cleaning of the ultrapure water supply system; and

[0027]FIG. 7 is a graph showing time-based changes in the number of fineparticles in the ultrapure water according to examples and a comparativeexample after the cleaning of a filtration membrane.

BEST MODE OF CARRYING OUT THE INVENTION

[0028] A cleaning method according to one embodiment of the presentinvention will be hereinafter described with reference to the drawings.

[0029] The cleaning method of this embodiment is applied to cleaning ofthe whole or a part (system component element such as a filtrationmembrane) of an ultrapure water supply system.

[0030] An ultrapure water supply system 1 to which the cleaning methodis applied includes, as shown in FIG. 1, an ultrapure water productionapparatus (secondary pure water production apparatus) 2 for obtainingultrapure water from primary pure water 10. The primary pure water 10 isobtained, for example, by passing raw water through a reverse osmosismembrane, then treating the water by using anion and cation exchangeresins in order, and further passing the water through a reverse osmosismembrane.

[0031] The ultrapure water production apparatus 2 is connected throughan ultrapure water supply passage 6 a to a cleaning apparatus 4 aarranged at a point 4 of use of ultrapure water. The cleaning apparatus4 a cleans objects to be cleaned, such as semiconductors, by using theultrapure water supplied from the ultrapure water production apparatus2. Namely, the ultrapure water supply system 1 of this embodiment issuited for supplying ultrapure water used in semiconductor manufacturingprocess.

[0032] The ultrapure water production apparatus (secondary pure waterproduction apparatus) 2 has a tank 21 for storing the primary pure water10. The tank 21 is connected via an ultrapure water return passage 6 bto the cleaning apparatus 4 a at the point 4 of use so that the part ofthe ultrapure water which was not used by the cleaning apparatus 4a mayreturn to the tank 21.

[0033] In the ultrapure water production apparatus 2, the pure water(the primary pure water 10 and the ultrapure water returned from thepoint 4 of use) in the tank 21 is fed by the action of a pump 22 to aheat exchanger 23 where the water temperature is adjusted, and then toan ultraviolet oxidation device 2 a where organic matter is removed fromthe pure water. Further, the pure water is treated in an ion exchangeresin tower 24 and then subjected to a final treatment in anultrafiltration membrane device 2 b where fine particles are removedfrom the pure water, thereby producing ultrapure water meeting qualityrequirements. In FIG. 1, reference numeral 30 denotes a bypass passagebypassing the ion exchange resin tower 24.

[0034] The ultrapure water is supplied from the ultrapure waterproduction apparatus 2 to the cleaning apparatus 4 a at the point 4 ofuse through the ultrapure water supply passage 6 a; part of theultrapure water is used by the cleaning apparatus while the unusedultrapure water is returned to the tank 21 of the ultrapure waterproduction apparatus 2 through the ultrapure water return passage 6 b.The ultrapure water used at the point 4 of use is collected aswastewater at the point 4 of use and is treated.

[0035] To explain the ultrapure water supply system in more detail, eachof the passages 6 a and 6 b basically comprises a pipe and may include atank, a pump, joints, valves and other devices arranged in the middlethereof.

[0036] The pipes (passages) 6 a and 6 b may be made of any materialinsofar as the constituents of the material used are not eluted from thepipes into the ultrapure water. For example, the pipes 6 a and 6 b maybe made of PVC (polyvinyl chloride), PPS (polyphenylene sulfide), PVDF(polyvinylidene difloride), FRP (fiber-reinforced plastic), PFA(tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), stainlesssteel, etc.

[0037] The cleaning method according to this embodiment will be nowdescribed.

[0038] The cleaning method is used for cleaning the ultrapure watersupply system, as mentioned above, or a component element thereof (e.g.,filtration membrane), and is characterized in that the surface potentialof fine particles present in the system is changed to facilitate theremoval of the fine particles.

[0039] First, the principle of removal of fine particles employed in thecleaning method will be explained.

[0040] Primarily due to the flow of liquid (pure water, ultrapure water,cleaning liquid, etc.) within the ultrapure water supply system 1, thesystem elements 2 a, 2 b, 4 a, 6 a, 6 b, 21 to 24 and 30 are chargedwith electricity and also fine particles inside the system 1 are chargedwith electricity. The polarity and magnitude of the surface potential ofthe fine particles are heavily dependent on the properties, pH inparticular, of the liquid, whereas the polarity and magnitude of thesurface potential of the system elements made of materials such as PVCare not so heavily dependent on the properties of the liquid.

[0041] The relationship between the surface potential and the pH of theliquid is illustrated in FIG. 2. FIG. 2 shows surface potentials of fourkinds of substances constituting the fine particles and of two kinds ofmaterials constituting the system elements, as a function of the pH. Thesurface potentials were measured by an electrophoretic light scatteringmeasurement method with the pH varied as indicated by individual marksin FIG. 2.

[0042] In FIG. 2, the solid lines indicate the surface potential-pHcharacteristics of the respective fine particle-constituting substancesand the dashed lines indicate the surface potential-pH characteristicsof the respective system-constituting materials. The marks of rhombus(♦), triangle (▴), square (▪) and circle () on the solid linescorrespond respectively to silica (SiO₂), alumina (Al₂O₃), polystylenelatex (PSL) and silicon nitrite (Si₃N₄). The marks of asterisk (*) andcircle () on the dashed lines correspond to PVC and PVDF, respectively.

[0043] As shown in FIG. 2, where the liquid has a pH value smaller than“7” and thus shows near neutrality or acidity, the surface potentials offine particles constituted by silica, alumina, PSL and Si₃N₄ take valuesclose to 0 mV or positive values. Where the liquid has a pH value largerthan “7” and shows alkalinity, the fine particle-constituting substanceshave negative surface potentials. On the other hand, the system elementsconstituted by PVC or PVDF show negative surface potentials throughoutthe whole pH range, regardless of the pH of the liquid.

[0044] The above fact indicates that in a certain situation, the surfacepotentials of the system elements and those of the fine particles haveopposite polarities, and in this case, the fine particles remain stuckon the surfaces of the system elements due to electrostatic attractiveforce produced between the system elements and the fine particles.

[0045] According to the cleaning method of this embodiment, an alkalineliquid, for example, is made to contact with the fine particles in thesystem to negatively charge the fine particles so that the surfacepotentials of the fine particles may be of the same polarity as those ofthe system elements, thereby eliminating the electrostatic attractiveforce acting between the fine particles and the system elements orproducing electrostatic repulsive force between the two. As a result,the fine particles can be easily removed from the system.

[0046] In the cleaning method based on the above principle, a cleaningliquid (chemical) having the effect of changing the surface potential isused. As a cleaning liquid having such an effect, basic solution ispreferred. The basic solution to be used is not particularly limitedinsofar as the solution is soluble in water and the pH thereof fallswithin a predetermined range, and may preferably be an aqueous solutionof ammonia or ammonium salt, for example. Other preferred basicsolutions include an aqueous solution of tetraalkylammonium compound ortetraalkylammonium salt, an aqueous solution of tetramethylammoniumhydroxide (TMAH), and an aqueous solution of a metal hydroxide such assodium hydroxide or potassium hydroxide. The basic solution mayalternatively be obtained by dissolving an alkaline gas such as ammoniagas in ultrapure water. The pH and concentration of the basic solutionare determined principally in order that the basic solution may havesatisfactory system cleaning capability and at the same time that theresidual constituent of the solution in the system may be small.

[0047] In the case of the aqueous solution of ammonia or sodiumhydroxide, the pH thereof is set to 7 to 14, preferably, to 9 to 11.

[0048] The concentration of the aqueous solution of ammonia for cleaningthe system is set to 5 to 500 mg/L, preferably, to a range of 50 to 100mg/L. The concentration of the aqueous solution of sodium hydroxide-forcleaning the system is set to 0.01 to 4000 mg/L, preferably, to a rangeof 0.4 to 40 mg/L.

[0049] The concentration of the aqueous solution of ammonia for cleaningthe filtration membrane is set to 20 to 2000 mg/L, preferably, to arange of 100 to 1000 mg/L, and the concentration of the aqueous solutionof sodium hydroxide for cleaning the filtration membrane is set to 0.4to 10,000 mg/L, preferably, to a range of 100 to 1000 mg/L.

[0050] The concentration of the aqueous solution of TMAH may be 10 to100 ppm, preferably, 40 to 60 ppm.

[0051] In the case of the basic solution obtained by dissolving analkaline gas in ultrapure water, ammonia gas, for example, is introducedinto the system so that the solution obtained may have a pH value and anammonia concentration falling within the above respective ranges. Theconcentration of the ammonia gas to be used is not particularly limitedand may be about 5 to 1000 ppm.

[0052] The following describes the manner of how the ultrapure watersupply system 1 of FIG. 1 is cleaned using the aforementioned basicsolution as the cleaning liquid.

[0053] First, a cleaning liquid 8, of which the pH and the concentrationare adjusted in advance so as to fall within the aforementionedrespective preferred ranges, is introduced into the tank 21.Alternatively, a basic salt, for example, is put into the tank 21storing the primary pure water 10 or the ultrapure water returned fromthe point of use, to thereby prepare the cleaning liquid 8 within thetank.

[0054] Subsequently, following an ordinary ultrapure water circulationpath which starts from the ultrapure water production apparatus 2,passes through the passage 6 a, the point 4 of use and the passage 6 b,and returns to the ultrapure water production apparatus 2, the cleaningliquid 8 is caused to circulate through the system 1 once or repeatedlyfor several hours, preferably, 0.5 to 3 hours. In this manner, the basicsolution as the cleaning liquid 8 is made to flow through thecirculation path, whereby the basic solution contacts with theindividual parts of the system 1, that is, the pump 22, the heatexchanger 23, the ultraviolet oxidation device 2 a, the ultrafiltrationdevice 2 b, the pipes connecting these devices, and the passages 6 a and6 b, and clean these devices and pipes.

[0055] During the cleaning, the flow velocity of the cleaning liquid 8is set to 0.5 m/sec or higher, preferably, to a value falling within arange of 0.75 to 2.0 m/sec.

[0056]FIG. 3 shows the relationship between the flow velocity of thecleaning liquid and the number of fine particles contained in ultrapurewater used for rinsing after the system cleaning. To obtain a flowvelocity-fine particle count curve as shown in FIG. 3, the number offine particles was measured after the cleaning liquid was caused to flowat each of flow velocities of 0.25, 0.75, 1.5 and 2 m/sec, as indicatedby the mark  in FIG. 3. Specifically, as in the case of the fineparticle count measurement performed in Example 1 described later,rinsing was carried out following the system cleaning by means of thecleaning liquid, and upon lapse of a whole day after the start of therinsing, the ultrapure water used for the rinsing was sampled andfiltered by means of a filter. Then, the number of fine particlestrapped on the filter was counted using a scanning electron microscope.As seen from FIG. 3, the cleaning effect obtained with the flow velocity0.25 m/sec is not of satisfactorily level, and the cleaning effect doesnot significantly improve and becomes almost saturated at a flowvelocity exceeding 2.0 m/sec. Accordingly, a preferred range of the flowvelocity is considered to be 0.5 to 2.0 m/sec.

[0057] In the case of cleaning a filtration membrane such as theultrafiltration membrane, the flow velocity of the cleaning liquid isset to 0.5 to 2.5 m/sec, preferably, 0.75 to 2.0 m/sec.

[0058] As the cleaning liquid 8 is caused to flow through the system 1in this manner, the cleaning liquid 8 comes into contact with fineparticles in the system 1, so that the polarity of the surface potentialof the fine particles changes from positive to negative while thenegative polarity of the surface potential of the component partsconstituting the system elements remains the same. Consequently, thesurface potential of the fine particles has the same polarity as thesurface potential of the system elements, with the result that theelectrostatic attractive force acting between the fine particles and thesystem elements is eliminated or electrical repulsive force is producedbetween the fine particles and the system elements, facilitating theremoval of the fine particles from the surfaces of the system elements.Further, the flow of the cleaning liquid 8 applies physical force to thefine particles, thus making it easier to remove the fine particles.Namely, the flow of the cleaning liquid 8 makes it possible or makes iteasier to separate fine particles adhering to the surfaces of the systemelements, for example.

[0059] The used cleaning liquid 8 which has been circulated once orrepeatedly through the system 1 is discharged from a blow pipe (notshown) of the system 1. At this time, the fine particles separated fromthe system elements and contained in the cleaning liquid 8 aredischarged from the system 1 to outside together with the cleaningliquid 8. The used cleaning liquid 8 is subjected to adsorption by meansof a weak acid cation exchange resin, for example.

[0060] According to the system cleaning method described above, even inthe case where the base or basic salt contained in the cleaning liquid 8has a low concentration of, for example, several tens of mg/L, thecleaning liquid 8 can produce a satisfactory effect ofseparating/removing the fine particles. If necessary, after the cleaningusing the cleaning liquid 8, ultrapure water is introduced into thesystem 1 to remove the residual constituent of the solution in thesystem.

[0061] In the case where the cleaning liquid 8 used has a lowconcentration, only a small amount of the basic constituent of thecleaning liquid 8 remains in the system 1, and this makes it possible toshorten the cleaning time for removing the residual constituent, wherebythe overall cleaning operation performed using the cleaning liquid 8 andthe ultrapure water can be completed in a short time. Thus, where theultrapure water supply system 1 is newly installed, only a short set-uptime is required. Also, in the subsequent production of ultrapure water,increase of TOC attributable to the basic constituent can be suppressed,contributing to the improvement in quality of the ultrapure waterproduced.

[0062] There are no particular restrictions on the temperature of thecleaning liquid 8, but from the point of view of cleaning capability,the temperature of the cleaning liquid should desirably be as high aspossible within a range such that the heat resistance temperatures ofthe parts and pipes constituting the ultrapure water supply system 1 arenot exceeded, and more specifically, is set to 20 to 100° C. Forexample, when cleaning the system or system elements made of PVC havinga heat resistance temperature of about 45° C., the temperature of thecleaning liquid 8 is set to approximately 40° C., and for the system orsystem elements made of PVDF having a heat resistance temperature ofabout 80° C., the temperature of the cleaning liquid is set to a valuenot higher than 75 to 80° C. In the case of the system or systemelements made of stainless steel, the cleaning liquid whose temperatureis adjusted to approximately 100° C. may be used.

[0063] While the cleaning method according to one embodiment of thepresent invention has been described, it is to be noted that the presentinvention is not limited to the foregoing embodiment alone and may bemodified in various ways.

[0064] For example, the ultrapure water supply system to which thecleaning method of the present invention is applied is not limited tothe one shown in FIG. 1, and it is not essential that the ultrapurewater supply system be constructed such that the ultrapure water whichwas not used at the point of use is returned to the ultrapure waterproduction apparatus. Also, a reverse osmosis membrane or otherfiltration devices, not shown in FIG. 1, may be incorporatedappropriately into the ultrapure water production apparatus 2. Further,the heat exchanger 23 and the ion exchange resin tower 24, appearing inFIG. 1, are not indispensable system component elements.

[0065] In the forgoing embodiment, the cleaning liquid is circulatedonce or repeatedly through the ultrapure water supply system so as tocontact with fine particles inside the system, thereby changing thesurface potential of the fine particles and also applying physical forceto the fine particles. Alternatively, with the ultrapure water supplysystem or part thereof filled with the cleaning liquid, the cleaningliquid may be applied with small-amplitude vibration caused byultrasonic waves, for example. In this case, the cleaning liquid filledin the system contacts with fine particles in the system, thus changingthe surface potential of the fine particles, and small-amplitudevibrations of the cleaning liquid are transmitted to the fine particles,thus applying physical force to the fine particles. It should be noted,however, that positively applying physical force to the fine particlesis not essential to the invention, and the cleaning liquid filled in thesystem may be merely discharged from the system after the system is leftat rest for a predetermined time with the cleaning liquid filledtherein.

[0066] Also, it is not essential in the present invention that the wholeultrapure water supply system be cleaned, and part of the system, forexample, the ultrafiltration membrane device 2 b, the ultravioletoxidation device 2 a, the pipes 6 a and 6 b and their joints, may beindividually cleaned. In this case, the cleaning liquid is introducedinto a to-be-cleaned part of the system from a location immediatelyupstream of the to-be-cleaned part and is discharged from a locationimmediately downstream of the to-be-cleaned part, thereby letting thecleaning liquid flow through the to-be-cleaned part. Instead of causingthe cleaning liquid to flow through the to-be-cleaned part, theto-be-cleaned part may be simply filled with the cleaning liquid, andafter a lapse of a predetermined time, the cleaning liquid may bedischarged from the to-be-cleaned part. Further, with the to-be-cleanedpart filled with the cleaning liquid, the cleaning liquid may bevibrated and then be discharged.

[0067] In this manner, the system can be cleaned with the systemelements, for example, the ultrafiltration membrane, attached to theultrapure water production apparatus, but the system elements may bedetached from the apparatus for cleaning. In this case, the cleaningliquid may be made to flow through a housing containing theultrafiltration membrane, or the housing may be simply filled with thecleaning liquid, with vibrations applied to the cleaning liquid asneeded.

[0068] Further, in the present invention, a surfactant, water in whichhydrogen gas is dissolved, or a mixture of the two, for example, may beused as the cleaning liquid in place of the basic cleaning liquid.Alternatively, electrode plates may be immersed in pure water togetherwith a to-be-cleaned part of the system, and with a separating membranearranged between the electrode plates, electric current is passedthrough the electrode plates, thereby to change the surface potential ofthe fine particles.

[0069] Where a surfactant is used as the cleaning liquid, cationsurfactant such as primary amine derivative or quaternary ammonium salt,or neutral surfactant of carboxylic acid type or sulfate type may beused. Also, an anion surfactant such as alkylbenzenesulfonate may beused in combination with basic salt. The concentration of the surfactantis set to 1 to 1000 mg/L, usually, to several tens of mg/L.

[0070] In order to enhance the cleaning effect, the cleaning liquid maybe introduced from the outlet of the ultrafiltration membrane 2 b intothe ultrapure water passage 6 a.

EXAMPLE 1

[0071] 1. Cleaning of Ultrapure Water Supply System

[0072] An ultrapure water supply system 1 as shown in FIG. 1 was newlyinstalled and was cleaned in the manner described below.

[0073] Ammonia water was added to the pure water in the tank 21 of theultrapure water production apparatus 2 to prepare a cleaning liquid 8having a concentration of 50 mg/L and a pH value of 10.5, and thecleaning liquid 8 was made to circulate at a flow velocity of 0.75 m/secthrough the system for two hours along the circulation path: ultrapurewater production apparatus 2→passage 6 a→point 4 of use→passage 6b→ultrapure water production apparatus 2. In this case, the cleaningliquid 8 was caused to flow through the bypass passage 30 bypassing theion exchange resin tower 24, and thus the ion exchange resin tower 24was not cleaned. Also, the temperature of the cleaning liquid 8 wasadjusted to 40° C. by the heat exchanger 23.

[0074] After the system was cleaned using the cleaning liquid 8, thecleaning liquid 8 was discharged from the blow pipe, not shown.Subsequently, the primary pure water 10 was introduced into the tank 21,and the ultrapure water produced by the ultrapure water productionapparatus 2 was circulated through the system 1 to remove the residualconstituent of the cleaning liquid remaining in the system. Theultrapure water containing the constituent of the cleaning liquid wasthen discharged from the system to outside.

[0075] 2. Evaluation of Cleaning Capability

[0076] After the constituent of the cleaning liquid was removedfollowing the system cleaning, the system 1 was operated under normalconditions, and the ultrapure water was sampled at the point 4 of use aplurality of times at given intervals of time. The sampled ultrapurewater was measured as to the number of fine particles contained and theTOC, to obtain a change with time in the quality of the ultrapure water.To measure the number of fine particles, a given amount of the ultrapurewater was passed through a filter, and the number of fine particles(with a particle diameter greater than or equal to 0.05 pm) trapped onthe filter was counted using a scanning electron microscope. The TOC wasmeasured by an ultraviolet oxidation-resistivity detection method.

[0077] The measurement results are shown in FIGS. 5, 6 and 7. The dashedline in the individual figures represents the required quality ofultrapure water used at the point of use.

EXAMPLE 2

[0078] Sodium hydroxide was added to the pure water in the tank 21 ofthe system 1 to prepare a cleaning liquid 8 having a concentration of 4mg/L and a pH value of 10.5, and the cleaning liquid 8 was circulatedthrough the system 1 in the same manner as in Example 1, to clean thesystem 1. A time-based change in the quality of the ultrapure waterproduced by the thus-cleaned system 1 was measured in the same manner asin Example 1. The measurement results are shown in FIGS. 5 and 6.

EXAMPLE 3

[0079] An ammonia gas inlet pipe 11 was added to the ultrapure watersupply system 1 shown in FIG. 1, thereby constructing a system 1 shownin FIG. 4.

[0080] Ammonia gas was introduced from the inlet pipe 11 into theultrapure water supplied from the tank 21, to obtain a cleaning liquid 8having an ammonia concentration of 50 mg/L and a pH value of 10.5. Then,in the same manner as in Example 1, the cleaning liquid 8 was circulatedthrough the system 1 to clean the system, and a time-based change in thequality of the ultrapure water produced by the cleaned system wasmeasured. The measurement results are shown in FIGS. 5 and 6.

EXAMPLE 4

[0081] An ultrafiltration membrane made of polysulfone was detached fromthe housing and was immersed in the same cleaning liquid 8 as used inExample 1 for two hours with ultrasonic waves applied to the cleaningliquid 8 to vibrate the same, thereby cleaning the ultrafiltrationmembrane. Subsequently, with the ultrafiltration membrane attached tothe housing, the ultrapure water was caused to flow through the housing,and the number of fine particles contained in the ultrapure water passedthrough the ultrafiltration membrane was measured by the same method asused in Examples 1 to 3. The measurement results are shown in FIG. 7.

Comparative Example 1

[0082] Using warm water of 40° C. as the cleaning liquid, the ultrapurewater supply system was cleaned in the same manner as in Examples 1 to3, and then the ultrapure water produced by the system was sampled atthe point of use to examine the water quality. The measurement resultsare shown in FIGS. 5 and 6.

Comparative Example 2

[0083] Using ultrapure water as the cleaning liquid, the ultrafiltrationmembrane was cleaned in the same manner as in Example 4, and then thenumber of fine particles in the ultrapure water produced by the cleanedsystem was measured. The measurement results are shown in FIG. 7.

Evaluation of Examples 1-5 and Comparative Examples 1-2

[0084] As is clear from FIG. 5, in the ultrapure water supply systemcleaned by the cleaning methods according to Examples 1 to 3, the numberof fine particles in the ultrapure water produced by the system droppedto a level lower than an allowable upper-limit value (1/mL) before awhole day passed since the start of operation of the system followingthe completion of the cleaning. Namely, the cleaning methods of Examples1 to 3 proved to have high fine particle removing capability. A cleaningmethod according to Example 5 also had excellent cleaning capability,though measurement results thereof are not illustrated. On the otherhand, in the system cleaned by the method according to ComparativeExample 1, more than eight days were required for the number of fineparticles in the ultrapure water produced by the system to drop to alevel lower than the allowable upper-limit value, and thus, it was foundthat the cleaning method of Comparative Example 1 had poor fine particleremoving capability.

[0085] Also, as seen from FIG. 6, in Examples 1 to 3, the time requiredfor the TOC of the ultrapure water produced by the system to drop to alevel lower than an allowable upper-limit value (1 μg/L) was shorterthan a whole day as counted from the start of operation of the system,proving that the cleaning methods of Examples 1 to 3 also have excellentorganic matter removing capability. This is the case with Example 5 aswell. On the other hand, in Comparative Example 1, it took four to fivedays for the TOC to decrease to a level lower than the allowableupper-limit value, and it was found that the cleaning method ofComparative Example 1 had poor organic matter removing capability.

[0086] Further, as is clear from FIG. 7, in Examples 1 and 4, the numberof fine particles decreased to a level lower than 1/mL before a wholeday passed since the start of operation of the system having the cleanedfiltration membrane attached thereto. Especially, in the case of Example4, the number of fine particles dropped to a level lower than 1/mL in 12hours. On the other hand, in Comparative Example 2, the number of fineparticles did not drop below 1/mL even after the lapse of 300 hours fromthe start of operation of the system.

1. A cleaning method for cleaning at least part of an ultrapure watersupply system having an ultrapure water production apparatus connectedto a point of use of ultrapure water via a passage, comprising the stepsof: (a) changing surface potential of fine particles present in the atleast part of the ultrapure water supply system; and (b) discharging thefine particles from the at least part of the ultrapure water supplysystem to outside.
 2. The cleaning method according to claim 1, whereinin said step (a), the fine particles are made to contact with a basicsolution or a solution of surfactant.
 3. The cleaning method accordingto claim 1, wherein in said step (a), the surface potential of the fineparticles is changed and also physical force is applied to the fineparticles.
 4. The cleaning method according to claim 3, wherein in saidstep (a), a basic solution or a solution of surfactant is caused to flowthrough the at least part of the ultrapure water supply system at a flowvelocity of 0.5 m/sec to 2.0 m/sec.
 5. The cleaning method according toclaim 3, wherein in said step (a), with a basic solution or a solutionof surfactant kept in contact with the at least part of the ultrapurewater supply system, the solution is applied with small-amplitudevibration.
 6. The cleaning method according to any one of claims 2, 4and 5, wherein the basic solution is an aqueous solution of ammonia orammonium salt, or an aqueous solution of alkali metal hydroxide, or amixture of the aqueous solution of ammonia or ammonium salt and theaqueous solution of alkali metal hydroxide.
 7. The cleaning methodaccording to any one of claims 2, 4 and 5, wherein the basic solution ispure water or ultrapure water in which alkaline gas is dissolved.